Karthus Wall of Pain slows spawning minions

Was playing an ARAM earlier, enemy team had a Karthus, if he would have Wall of Pain down when our minions were spawning from the nexus they would be affected by the slow from his Wall. It would only happen on the minions that were spawning out of the nexus while the wall was down, it didnt matter where the wall was it would just globally slow any minions as the came out.

Sadly I dont have the time atm to try and recreate it but this would happen every time wall was down and our minions were spawning.